The present invention pertains to the art of x or gamma ray generation. It finds particular application in conjunction with x-ray tubes for CT scanners and will be described with particular reference thereto. However, it is to be appreciated, that the present invention will find application in conjunction with the generation of x-rays for other applications.
Typically, a patient is positioned in a prone position on a horizontal couch through a central bore of a CT scanner. An x-ray tube is mounted on a rotatable gantry portion and rotated around the patient at a high rate of speed. For faster scans, the x-ray tube is rotated more quickly. However, rotating the x-ray more quickly decreases the net radiation per image. As CT scanners have become quicker, larger x-ray tubes which generate more radiation per unit time have been required, which, of course, cause high inertial forces.
High performance x-ray tubes for CT scanners and the like commonly include a stationary cathode and a rotating anode disk, both enclosed within an evacuated housing. As stronger x-ray beams are generated, there is more heating of the anode disk. In order to provide sufficient time for the anode disk to cool by radiating heat through the vacuum to surrounding fluids, x-ray tubes with progressively larger anode disks have been built.
The larger anode disk requires a larger x-ray tube which does not readily fit in the small confined space of an existing CT scanner gantry. Particularly in a fourth generation scanner, incorporating a larger x-ray tube and heavier duty support structure requires moving the radiation detectors to a larger diameter. This requires more detectors for the same resolution and provides a longer path length between the x-ray tube and the detectors. The longer path length can cause more radiation divergence and other degradation of the image data. Not only is a larger x-ray tube required, larger heat exchange structures are required to remove the larger amount of heat which is generated.
Rather than rotating a single x-ray tube around the subject, others have proposed using a switchable array of x-ray tubes, e.g. five or six x-ray tubes in a ring around the subject. However, unless the tubes rotate only limited data is generated and only limited image resolution is achieved. If the x-ray tubes rotate, similar mechanical problems are encountered trying to move all the tubes quickly.
Still others have proposed constructing an essentially bell-shaped, evacuated x-ray tube envelope with a mouth that is sufficiently large that the patient can be received in the well of the tube. An x-ray beam source is disposed at the apex of the bell to generate an electron beam which impinges on an anode ring at the mouth to the bell. Electronics are provided for scanning the x-ray beam around the evacuated bell-shaped envelope. One problem with this design is that it is only capable of scanning about 270.degree. . Another problem is that the very large evacuated space required for containing the scanning electron beam is difficult to maintain in an evacuated state. Troublesome and complex vacuum pumping systems are required. Another problem is that no provision can be made for off-focus radiation. Another problem resides in its large physical size.
Messrs. Mayden, Shepp, and Cho in "A New Design For High-Speed Computerized Tomography", IEEE Transactions on Nuclear Science, Vol. NS-26, No. 2, Apr. 1979, proposed reducing the size of the conical or bell-shaped tube discussed above by rotating the cathode around the large diameter anode ring. However, their design had several engineering deficiencies and was never commercially produced.
The present invention contemplates a new and improved x-ray tube which can provide a tenfold or better power increase over currently available rotating anode x-ray tubes.